This invention relates to composite polymeric materials, particularly composite materials containing rod-like aromatic heterocyclic polymers dispersed in a polymeric matrix.
Chopped fiber reinforced plastics are currently being used in the fabrication of a wide variety of components. There are several disadvantages in the use of fiber for the reinforcement of plastic. In the case of chopped glass fibers, a large amount of fiber, generally a minimum of 30 percent by weight, is necessary for reinforcement because of the low reinforcing effect of the fiber. There is a practical processing limit on the effective fiber length. A macroscopically long fiber length is required with due regard for breaking or destruction of the fiber during processing, particularly molding. Composite materials containing chopped fibers are generally less processable than their non-reinforced counterparts. The shape of moldings is often limited to simple block or sheet forms. Films or filaments cannot be formed from chopped glass fiber-reinforced plastics. Other disadvantages of these materials include poor surface properties of molded articles, an anisotropy in dynamic properties, molding defects due to heterogeneity of the polymeric materials, and low cycle time in processing.
A need exists for high strength reinforced composites and a method for their manufacture which possess at least the following desirable prerequisites: (1) non-reliance on fiber reinforcement for the attainment of high strength properties; (2) circumvention of the complexities of current composite fabrication procedures; and (3) elimination of any possibility of fiber-polymer interface problems.
Various attempts have been made to overcome some of the above-described disadvantages of chopped-fiber reinforced plastics. One approach described by Helminiak et a., U.S. Pat. Nos. 4,207,407 and 4,377,546, comprises the dispersion of an intrinsically rigid rod-like heterocyclic polymer in a flexible, coil-like heterocyclic polymer.
The above composites are referred to as molecular composites. While this approach represents a valuable contribution to the art, it has certain drawbacks. For example, poly(p-phenylene benzobisthiazole) (PBT) has superior mechanical properties and thermal stability. However, PBT degrades before it melts; therefore, processing of a composite containing PBT must be carried out in a solution state with an acid, such as methanesulfonic acid (MSA), as the solvent. Relatively few flexible coil polymers can be dissolved in or are stable in MSA, thus limiting the choice of matrix polymers. Molecular composites based on PBT and poly-2,5-benzimidazole (ABPBI) have been fabricated into fibers and thin films. However, ABPBI does not have a glass transition temperature (T.sub.g). Therefore, molecular composites containing ABPBI are difficult to thermally consolidate into thicker specimens. To overcome this problem, thermoplastic matrices have been used so that the molecular composite films could be laminated. However, thicker specimens fabricated using thermoplastic matrices are limited to use at temperatures below the T.sub.g of the matrix polymer(s). Conventional thermoset resins, such as bismaleimides, epoxies and the like, are not stable in the acid medium used to process the rigid-rod polymer, and cannot be used as host matrices for molecular composites.
A drawback to molecular composites based upon rod-like and coil-like aromatic heterocyclic polymers has to do with phase separation between the two polymers. Such separation can occur during coagulation/precipitation and/or during consolidation. During coagulation/precipitation the polymers can separate due to insufficient entanglement on the part of the coil-like polymer, lack of strong rod-coil specific interaction or differences in rates of coagulation. Phase separation can also be thermally induced during consolidation.
One solution to the problem of phase separation is provided by Tan et al, U.S. Pat. No. 5,086,120 which discloses a molecular composite system consisting essentially of a para-oriented benzobisazole polymer and poly(2-acrylamido-2-methylpropanesulfonic acid). This molecular composite system can be processed into fibers and films with little or no detactable phase separation.
A drawback to molecular composites based upon rod-like aromatic heterocyclic polymers and thermoplastic or thermosetting resins has to do with the propensity of the rod-like materials to agglomerate. Serious agglomeration can lead to structural failure. Minor agglomeration can often be accommodated. What is desired is a molecular composite system in which there is interaction between the matrix polymer and the rod-like polymer sufficient to overcome any tendency toward phase separation.
Accordingly, it is an object of this invention to provide a novel molecular composite system.
Other objects, aspects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the invention.